This invention relates to a new and improved method of sanitizing, and simultaneously regenerating, backwashable, packed or fixed bed adsorption filters formed of porous media or materials which are typically employed during the purification of municipal or industrial water and wastewater streams. More specifically, the invention relates to sanitizing and regenerating porous particles comprising activated carbon that are exhausted by the proliferation of microorganisms, organics and/or other contaminants.
Examples of porous filter media include granular activated carbon (or “GAC”) or pelletized activated carbon, activated alumina, zeolite and synthetic magnesium silicate. Porous filter media such as granular activated carbon is widely used in water treatment systems to filter organic material, and to improve taste and odor. Activated carbon is also used in air treatment systems to remove volatile organic carbon (VOC). Depending on the water to be treated, GAC fixed bed adsorption filters may be used in combination (i.e. in sequence) with a primary filtration bed comprised of a different filter media (e.g. sand or other dual, or “mixed,” filter media).
Activated carbon is one of the most powerful adsorbents. It has a high internal porosity, and hence a large internal surface area (i.e. 500-1500 m2/g). As such, it has the ability to remove a large variety and wide range of compounds from contaminated waters and/or air, including organics. This makes it one of the most effective media for removing a wide range of contaminants from surface water, industrial and municipal waste waters, landfill leachate and contaminated groundwater, which has led to its increased use for water treatment. Adsorption is particularly effective in treating low concentration waste streams and in meeting stringent treatment levels.
One limitation in the use of granular activated carbon to filter water, however, is that it provides a breeding ground for bacterial growth when the activated carbon is used to remove organic contaminants as this adsorbed organic material is used as food for the bacteria. In addition, GAC deteriorates residual disinfectants, such as chlorine and ozone, by chemical reduction.
These two factors can lead to excessive bacterial growth in a filter bed when microbes are present, which can in turn lead to serious problems with either the treatment system (i.e. increased head loss across the filter, more frequent backwashing or “down time”, etc) or with the filtered, or treated effluent stream (presence of opportunistic pathogens, unwanted taste and odors, increased turbidity, etc.).
Bacterial growth, therefore, in activated carbon filter media used for the removal of organic contaminants in water can be a serious problem. To avoid bacterial growth requires periodic sanitization of the filter to control growth and the resulting contamination of the filtered effluent stream. For example, weekly sanitization cycles of up to 24 hours are often required. Sanitization is usually done with steam, with significant carbon loss, or chemicals (e.g. caustic soda), which requires extensive rinsing. Both of these processes require having the filter out of service for many hours, are costly in terms of capital investment and energy consumption, and have a negative environmental impact. Furthermore, these conventional sanitization processes often give limited, if any, restoration, or recuperation, of the carbon's adsorptive capacity (“regeneration”), as discussed below.
Another limitation in the use of activated carbon for filtering water of any type (e.g. wastewater, industrial process water, surface water, drinking water, groundwater, etc.) is the exhaustion of the carbon's adsorptive capacity over time, which in turn interferes with the filter's ability to function. After a period of filtration, the pores of the porous media particles begin to accumulate contaminants (or impurities) that can inhibit the filter's operation. These contaminants can be biological (i.e. viruses, bacteria, protozoans, and other microorganisms) or non-biological, (organics, chlorine, arsenic), and because they are adsorbed, they cannot be removed by conventional backwashing (which removes loose particles contained within the filter bed). The adsorbed materials fill the pores and cover the surface of the porous carbon particles contained within the filter, which decreases or “exhausts” the adsorption capacity of the carbon and results in reduced filter performance (e.g. higher backwash frequency, reduced flow-rates, increased water turbidity, breakthrough of contaminants or a combination thereof). Therefore, periodic replacement or regeneration, either onsite or off-site, of the carbon is required.
Activated carbon is an expensive filter media, and in many cases, the cost of replacing saturated (or “spent”) carbon is too expensive for its use to be economically feasible. The cost depends on how frequent carbon replacement is required, which is related to the contaminant load within the water stream. There is an additional cost to replacement because it can require significant filter downtime and labor, and it may not be possible for water treatment systems that only have one filtration bed. As an alternative to full replacement, the exhausted carbon can be regenerated either onsite or off site to restore the adsorptive capacity of spent activated carbon by desorbing, or removing, adsorbed contaminants.
Several methods exist for the regeneration of spent activated carbon:
Thermal Regeneration. This is the most common technique employed in industrial processes to regenerate activated carbon. Using this process the activated carbon is heated to a specified temperature, which then burns off the adsorbed organic contaminants. This is a widely used method and regenerates the carbon very well, but it has disadvantages. First, thermal regeneration causes high carbon losses. Second, because it requires high temperatures, it requires a considerable capital expense for a high energy roasting furnace, and the process is an environmentally and financially expensive process. Water treatment plants that use thermal regeneration, therefore, must be large enough to make it economically viable for regeneration to occur onsite. For smaller treatment plants, it is more common to ship spent activated carbon to a specialized regeneration facility. However, this requires unpacking the filter, transporting it to a facility (often times remotely located), returning the carbon to the plant, and repacking the filter. As with replacement, thermal regeneration has the additional cost associated with filter downtime.
Steam Regeneration/Sanitization. This method can be employed on its own or in combination with thermal regeneration. On its own, this method is limited to regenerating carbon which has retained only a few, highly volatile, contaminants. Furthermore, in practice, cooling the carbon after steam sanitization tends to fracture the carbon granules producing fines. This causes carbon loss and requires expensive polishing filters positioned immediately after the carbon filters, wherein the polishing filters need to be replaced every few weeks.
Alternative Methods. Alternative methods of regenerating and sanitizing activated carbon are needed due to the high cost, environmental impact and limitations of conventional methods. Known alternative regeneration and sanitization methods include: chemical and solvent regeneration; electrochemical regeneration; ultrasonic regeneration; and wet air oxidation. Many of these methods, if not all, have not yet been applied on an industrial scale.
Thus, a need exists for improved methods for the sanitization and/or regeneration of granular porous filter media, such as GAC. More specifically, there is an unfilled need for improved, on-site sanitization methods that are faster; simple; have lower energy costs; have reduced environmental impact; are easier to implement in situ (or “in place”) without the need to unpack a filter bed and transport the carbon to an off-site facility; and result in sanitization and regeneration at a level acceptable on an industrial scale.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicant in no way disclaims these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was, at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.